sp-runtime-interface add table about FFI types (#4668)

Adds a table to the rustdoc that shows how each individual type is
passed between the wasm and the host side.

primitives/runtime-interface/src/impls.rs

@@ -150,7 +150,7 @@ impl IntoFFIValue for bool {
 ///
 /// The `u64` value is build by `length 32bit << 32 | pointer 32bit`
 ///
-/// If `T == u8` the length and the pointer are taken directly from the `Self`.
+/// If `T == u8` the length and the pointer are taken directly from `Self`.
 /// Otherwise `Self` is encoded and the length and the pointer are taken from the encoded vector.
 impl<T> RIType for Vec<T> {
 	type FFIType = u64;
@@ -209,7 +209,7 @@ impl<T: 'static + Decode> FromFFIValue for Vec<T> {
 ///
 /// The `u64` value is build by `length 32bit << 32 | pointer 32bit`
 ///
-/// If `T == u8` the length and the pointer are taken directly from the `Self`.
+/// If `T == u8` the length and the pointer are taken directly from `Self`.
 /// Otherwise `Self` is encoded and the length and the pointer are taken from the encoded vector.
 impl<T> RIType for [T] {
 	type FFIType = u64;
@@ -400,7 +400,7 @@ for_primitive_types! {
 ///
 /// The `u64` value is build by `length 32bit << 32 | pointer 32bit`
 ///
-/// The length and the pointer are taken directly from the `Self`.
+/// The length and the pointer are taken directly from `Self`.
 impl RIType for str {
 	type FFIType = u64;
 }

primitives/runtime-interface/src/lib.rs

@@ -28,7 +28,7 @@
 //! implement [`RIType`]. The associated type `FFIType` is the type that is used in the FFI
 //! function to represent the actual type. For example `[T]` is represented by an `u64`. The slice
 //! pointer and the length will be mapped to an `u64` value. For more information, see the
-//! implementation of [`RIType`] for [`T`]. The FFI function definition is used when calling from
+//! implementation of [`RIType`] for `T`. The FFI function definition is used when calling from
 //! the wasm runtime into the node.
 //!
 //! Traits are used to convert from a type to the corresponding [`RIType::FFIType`].
@@ -69,6 +69,34 @@
 //!
 //! For more information on declaring a runtime interface, see
 //! [`#[runtime_interface]`](attr.runtime_interface.html).
+//!
+//! # FFI type and conversion
+//!
+//! The following table documents how values of types are passed between the wasm and
+//! the host side and how they are converted into the corresponding type.
+//!
+//! | Type | FFI type | Conversion |
+//! |----|----|----|
+//! | `u8` | `u8` | `Identity` |
+//! | `u16` | `u16` | `Identity` |
+//! | `u32` | `u32` | `Identity` |
+//! | `u64` | `u64` | `Identity` |
+//! | `i8` | `i8` | `Identity` |
+//! | `i16` | `i16` | `Identity` |
+//! | `i32` | `i32` | `Identity` |
+//! | `i64` | `i64` | `Identity` |
+//! | `bool` | `u8` | `if v { 1 } else { 0 }` |
+//! | `&str` | `u64` | <code>v.len() 32bit << 32 &#124; v.as_ptr() 32bit</code> |
+//! | `&[u8]` | `u64` | <code>v.len() 32bit << 32 &#124; v.as_ptr() 32bit</code> |
+//! | `Vec<u8>` | `u64` | <code>v.len() 32bit << 32 &#124; v.as_ptr() 32bit</code> |
+//! | `Vec<T> where T: Encode` | `u64` | `let e = v.encode();`<br><br><code>e.len() 32bit << 32 &#124; e.as_ptr() 32bit</code> |
+//! | `&[T] where T: Encode` | `u64` | `let e = v.encode();`<br><br><code>e.len() 32bit << 32 &#124; e.as_ptr() 32bit</code> |
+//! | `[u8; N]` | `u32` | `v.as_ptr()` |
+//! | `*const T` | `u32` | `Identity` |
+//! | [`T where T: PassBy<PassBy=Inner>`](pass_by::Inner) | Depends on inner | Depends on inner |
+//! | [`T where T: PassBy<PassBy=Codec>`](pass_by::Codec) | `u64`| <code>v.len() 32bit << 32 &#124; v.as_ptr() 32bit</code> |
+//!
+//! `Identity` means that the value is converted directly into the corresponding FFI type.
 
 #![cfg_attr(not(feature = "std"), no_std)]
 

primitives/runtime-interface/src/pass_by.rs

@@ -14,11 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
-//! Provides the [`PassBy`](pass_by::PassBy) trait to simplify the implementation of the
+//! Provides the [`PassBy`](PassBy) trait to simplify the implementation of the
 //! runtime interface traits for custom types.
 //!
-//! [`Codec`](pass_by::Codec), [`Inner`](pass_by::Inner) and [`Enum`](pass_by::Enum) are the
-//! provided strategy implementations.
+//! [`Codec`], [`Inner`] and [`Enum`] are the provided strategy implementations.
 
 use crate::{RIType, util::{unpack_ptr_and_len, pack_ptr_and_len}};